In the Nothing Else Matters tab by Metallica on Songsterr.com, found here, I came across some notation that I haven't seen before:
Harm. mean in the picture above?
Harm. Just means to play the harmonics at that fret. The diamond is also the same instruction of playing harmonics at the 12th fret.
The 12 feet has very strong harmonics that are the same note as the string played on, just at higher octaves. Harmonics in general also have a very distinct tone.